1. Field of the Invention
The present invention relates to a method of preparing nano scale nickel powders by wet reducing process, and more particularly, a method of preparing nickel powders having minute and uniform particle sizes with a low production cost and high productivity.
2. Description of the Related Art
Nickel powders can be used as an inner electrode material of MLCC (multi layer ceramic capacitor) and an inner electrode material or a wiring material of other electric apparatuses.
The MLCC is an electrical apparatus transiently storing charges. Such MLCC has a structure that has many ceramic dielectric layer and flat electrode layers laminated on the ceramic dielectric layer.
The MLCC having such a structure is widely used in electronic devices, such as a computer and a mobile communication device, since it can obtain high capacitance with only a small volume.
Recently, there is a tendency to replace palladium (Pd) which was used as an electrode material of MLCC with nickel (Ni), which is inexpensive, to lower the cost of the MLCC. Thus, the inner electrode layer of the MLCC is formed with an electrode paste, which comprises nickel powders, by screen-printing.
To minimize the size of the MLCC and increase capacitance, the inner electrode layers having a thin thickness, i.e., a thickness less than 0.5 μm must be formed, and the techniques of preparing the electrode paste therefore are required. Further, in order to prepare a paste to form a thin electrode layer, nickel powders that are nano-scale and good in dispersity are required.
Research on the preparation of nano-scale nickel powders has been performed for a long time. The preparation method thereof includes a gas-state method and a liquid-state method.
The gas-state method is widely used since the shape of nickel powders, and impurities are relatively easily controllable. However, the method has disadvantages in the minimization of particles and the mass production. Meanwhile, the liquid-state method has advantages in that it is useful in mass production, the initial investment cost is low, and the process cost is low.
The representative example of the liquid-state method is a method of preparing a metal powder using a polyol. The method is described in U.S. Pat. No. 4,539,041.
U.S. Pat. No. 4,539,041 proposes a method of preparing a metal powder comprising dispersing a metal element, such as gold, platinum, silver, nickel, etc., in the form of a hydroxide, an oxide or a salt, into a liquid-state polyol reducing agent to prepare a mixture, and heating the mixture.
Experimentally, it was found that the pH range of the mixture, in which the metal compound is most easily reduced by a polyol, is about 9 to 11.
Thus, in the method of preparing nickel powders according to the prior art polyol method, an inorganic base, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), etc. was added to the mixture of a polyol and a nickel compound to maximize the reduction effect of the polyol, and the polyol was used as a solvent for the inorganic base. That is, the major function of the inorganic base is to control the pH of the mixture to a proper level.
However, since a polyol is expensive and its solubility is low, when the polyol was used as a solvent for the inorganic base, it contributed to the rise in the cost of preparing nickel powders.
Further, since the method of preparing nickel powders according to the prior art polyol method has problems of low yield, low degree of spheres and large particle size due to the non-uniform distribution of particle size, an improvement in the method is desired.
Accordingly, a method of preparing nickel powders having minute and uniform particle sizes with a low production cost and high productivity is desired.